LED Semiconductor Element Having Increased Luminance

ABSTRACT

An LED semiconductor element comprising at least one first radiation-generating active layer and at least one second radiation-generating active layer which is stacked above the first active layer in a vertical direction and is connected in series with the first active layer, wherein the first active layer and the second active layer are electrically conductively connected by means of a contact zone.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/441,758, which was filed with the U.S. Patent and Trademark Office onMar. 18, 2009 and which is a U.S. national stage under 35 USC §371 ofPCT Application No. PCT/DE2007/001535, filed on Aug. 28, 2007, claimingpriority from German Application No. 10 2006 046 039.1, filed on Sep.28, 2006, and German Application No. 10 2006 051 745.8, filed on Nov. 2,2006. The entire disclosures of each of these applications are expresslyincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an LED semiconductor element and to the use ofan LED semiconductor element.

BACKGROUND OF THE INVENTION

A high luminance is desirable for optical applications such asprojection applications or display backlightings. In conventional LEDsemiconductor elements, the amount of radiation generated depends on thecurrent intensity with which the LED semiconductor element is operated.However, the current density in the active layer should not exceed amaximum current density dependent on the semiconductor material used,since otherwise there is the risk of excessive ageing effectsdisadvantageously shortening the lifetime of the LED semiconductorelement.

SUMMARY OF THE INVENTION

It is an object of the present invention to specify an LED semiconductorelement having an increased luminance.

This and other objects are attained in accordance with one aspect of thepresent invention directed to an LED semiconductor element comprising atleast one first radiation-generating active layer and at least onesecond radiation-generating active layer which is stacked above thefirst active layer in a vertical direction and is connected in serieswith the first active layer, wherein the first active layer and thesecond active layer are electrically conductively connected by means ofa contact zone.

In the present case, the contact zone should be understood to be aregion of comparatively good electrical conductivity, wherein thecontact zone is preferably embodied in a manner free of tunnel contactsand therefore does not constitute a tunnel junction. Moreover, in theLED semiconductor element according to an embodiment of the invention,no tunnel junction is required for a charge carrier transfer between thefirst and the second active layer. This has the advantage that the LEDsemiconductor element can also be produced from materials with which atunnel junction is relatively difficult to realize epitaxially. Althoughthe active layers could be connected in parallel, such that a tunneljunction would be superfluous, a parallel connection would have thedisadvantage that with different series resistances, the same currentcould not be injected into the two active layers, or could be injectedonly with considerable additional outlay. It is advantageously possibleaccording to an embodiment of the invention to provide for a sufficientcharge carrier transfer between the first and the second active layer bymeans of the contact zone and furthermore to inject the same currentinto the two active layers by means of the series connection.

In the present series connection, the pn junctions of the active layersare preferably arranged in the same sense, such that they form a pn-pnor np-np structure. It goes without saying that in the case of more thantwo active layers, a pn . . . pn or np . . . np structure is preferred.

Apart from a simple pn junction, the active layers can have a doubleheterostructure, a single quantum well or a multiple quantum well (MQW)structure. Examples of MQW structures are described in the documents WO01/39282, WO 98/31055, U.S. Pat. No. 5,831,277, EP 1 017 113 and U.S.Pat. No. 5,684,309, the disclosure content of all of which concerningthe MQW structures is hereby incorporated by reference.

In particular, two arrangements of the contact zone are preferred in thecontext of embodiments of the invention. In accordance with a firstarrangement, the contact zone is arranged at a side flank of thesemiconductor element. In accordance with a second arrangement, thecontact zone is integrated into the LED semiconductor element betweenthe first active layer and the second active layer.

Since the first active layer and the second active layer are connectedin series, in the case of the active layers being arranged in the samesense, the contact zone expediently connects a semiconductor layer of afirst conductivity type to a semiconductor layer of a secondconductivity type. Preferably, the semiconductor layer of the firstconductivity type is disposed downstream of the first active layer in avertical direction, while the semiconductor layer of the secondconductivity type is arranged in a vertical direction between thesemiconductor layer of the first conductivity type and the second activelayer. By way of example, the semiconductor layer of the firstconductivity type can be a p-doped semiconductor layer and thesemiconductor layer of the second conductivity type can be an n-dopedsemiconductor layer. As an alternative, the semiconductor layer of thefirst conductivity type can be an n-doped semiconductor layer and thesemiconductor layer of the second conductivity type can be a p-dopedsemiconductor layer. This depends on the arrangement of the pn junctionsof the active layers.

In order to improve the charge carrier transfer in a semiconductorelement whose contact zone is arranged at the side flank, thesemiconductor layer of the first conductivity type and the semiconductorlayer of the second conductivity type can form a tunnel junction thatsupports the charge carrier transfer in addition to the contact zone. Inparticular, the semiconductor layer of the first conductivity type andthe semiconductor layer of the second conductivity type can be highlydoped for this purpose.

In accordance with one preferred embodiment, the semiconductor layer ofthe first conductivity type comprises a first free region not covered bysemiconductor material. With further preference, the semiconductor layerof the second conductivity type comprises a second free region notcovered by semiconductor material. In particular, the semiconductorlayer of the second conductivity type can project with respect to therest of the semiconductor element, while the semiconductor layer of thefirst conductivity type projects with respect to the layer of the secondconductivity type. Consequently, the form of the semiconductor elementbetween the first active layer and the second active layer in crosssection can correspond to a stepped form at least at a side flank. Itshould be pointed out that the LED semiconductor element has a layersequence of layers, of which at least a portion contains a semiconductormaterial. In the present case, the free region not covered bysemiconductor material should be understood to be a region not coveredby a semiconductor material used for the layers of the layer sequence.

In accordance with a particularly preferred variant, the contact zoneextends from the first free region to the second free region. Inparticular, the contact zone can be a contact layer. By way of example,the contact layer can at least partly cover the first free region andthe second free region.

Materials used for the contact zone and dimensions of the contact zoneare preferably chosen depending on the lateral conductivity of thelayers which the contact zone electrically conductively connects. By wayof example, a p-doped GaN layer has a relatively low lateralconductivity, for which reason the contact zone in this case should beembodied in comparatively large-area fashion and should contain amaterial having high electrical conductivity.

In accordance with one preferred embodiment of the LED semiconductorelement, the contact zone contains a metallic material. Such a contactzone is distinguished by a comparatively good electrical conductivity.This advantageously facilitates the charge carrier transfer between thefirst active layer and the second active layer.

In an alternative or more extensive configuration of the LEDsemiconductor element, the contact zone can be formed from a TCO(transparent conductive oxide) such as indium oxide, tin oxide, indiumtin oxide (ITO) or zinc oxide.

A contact zone containing a TCO is advantageouslyradiation-transmissive, such that the radiation generated in a regionbelow the contact zone can be coupled out from the semiconductor elementthrough the contact zone.

In accordance with one advantageous configuration, the first and thesecond active layer are monolithically integrated in the semiconductorelement. In this case, the first and the second active layer can beproduced in a common production step.

Furthermore, the semiconductor element in an embodiment of the inventioncan be a thin-film semiconductor element. If the semiconductor elementis composed of prefabricated layer stacks, then the individual layerstacks can be thin-film semiconductor bodies. A thin-film semiconductorelement is distinguished in particular by at least one of the followingcharacteristic features:

-   -   a reflective layer is applied or formed at a first main        area—facing toward a carrier element—of a radiation-generating        epitaxial layer sequence, said reflective layer reflecting at        least part of the electromagnetic radiation generated in the        epitaxial layer sequence back into the latter;    -   the epitaxial layer sequence has a thickness in the range of 20        μm or less, in particular in the range of between 2 μm and 10        μm; and    -   the epitaxial layer sequence contains at least one semiconductor        layer having at least one area having an intermixing structure        which ideally leads to an approximately ergodic distribution of        the light in the epitaxial layer sequence, that is to say that        it has an as far as possible ergodically stochastic scattering        behavior.

The basic principle of a thin-film light emitting diode chip isdescribed for example in I. Schnitzer et al., Appl. Phys. Lett. 63 (16),Oct. 18, 1993, 2174-2176, the disclosure content of which in thisrespect is hereby incorporated by reference.

A thin-film semiconductor element is, to a good approximation, aLambertian surface emitter and is therefore particularly well suited toprojection applications.

As already mentioned, in the case of the second arrangement the contactzone is integrated into the LED semiconductor element between the firstactive layer and the second active layer.

In accordance with one preferred configuration, the semiconductorelement comprises a first layer stack including the first active layer,and a second layer stack including the second active layer. Particularlypreferably, the contact zone is embedded between the first layer stackand the second layer stack in the case of this configuration.

In particular, the first layer stack comprises a semiconductor layer ofa first conductivity type in addition to the first active layer and thesecond layer stack comprises a semiconductor layer of a secondconductivity type in addition to the second active layer. Preferably,the first and the second layer stack are produced from two individualwafers. In order to produce the semiconductor element according to anembodiment of the invention, the wafers can be bonded onto one anotherin such a way that the semiconductor layer of the first conductivitytype and the semiconductor layer of the second conductivity type faceone another.

In accordance with a configuration to which further preference is given,the contact zone is arranged between the semiconductor layer of thefirst conductivity type and the semiconductor layer of the secondconductivity type. Consequently, the contact zone is arranged in themain beam path of the semiconductor element, while in the firstarrangement the contact zone is arranged in particular outside the mainbeam path.

In accordance with one preferred embodiment, the contact zone is acontact layer.

In accordance with an embodiment to which further preference is given,the contact zone has at least one first region and at least one secondregion. Particularly preferably, the second region is electricallyconductive. The first region can be electrically conductive orinsulating. By way of example, the contact zone can comprise a secondregion in the form of a contact pad or elongated contact web, wherein amaterial surrounding the second region forms the first region. Thesecond region is advantageously arranged in such a way that it producesan electrical connection between the first and the second layer stack.In particular, the second region can contain a metallic material. Thesecond region is preferably applied on a surface of the first or secondlayer stack which faces the opposite layer stack. As an alternative,each layer stack can have at least one second region arranged in such away that in each case two second regions come to lie on one another whenthe two layer stacks are stacked one on another.

The contact zone expediently contains a material that is transmissive tothe radiation generated by the first and/or second active layer.Consequently, there is no need to fear significant radiation lossesthrough the contact zone arranged in the main beam path.

The contact zone can contain a TCO. Furthermore, the contact zone cancontain an adhesion agent.

Furthermore, a first connecting layer can be applied on thesemiconductor layer of the first conductivity type and a secondconnecting layer can be applied on the semiconductor layer of the secondconductivity type. The connecting layers can be provided in particularfor further improving the charge carrier transfer between the layerstacks. Preferably, the connecting layers contain aradiation-transmissive and electrically conductive material such as TCO.Particularly preferably, the contact zone is arranged between the firstconnecting layer and the second connecting layer.

Furthermore, a mechanical connection is advantageously produced by meansof the contact zone between the first and second layer stacks.

Preferably, the first and the second active layer generate radiationhaving the same wavelength. The amount of radiation is thusadvantageously increased by comparison with conventional LEDsemiconductor elements.

With further preference, the main emission from the LED semiconductorelement is effected in a vertical direction. In particular, the mainemission is effected within a comparatively constricted solid angle,such that the luminance is advantageously increased. The luminance isthe optical power per emission area of the semiconductor element andsolid angle element.

Particularly preferably, the radiation generated by the first activelayer radiates through the second active layer. This is advantageousparticularly in combination with a reflection layer that can be providedfor the reflection of the radiation generated by the active layers in avertical direction. For in contrast to active layers which generateradiation of different wavelengths, in this case the absorption ofreflected radiation by the respective other active layer has nodisadvantageous effect on the total radiation emitted.

In accordance with one variant, the semiconductor element, preferablyone of the two active layers or both active layers, containsAl_(n)Ga_(m)In_(1-n-m)P, where 0≦n≦1, 0≦m≦1 and n+m≦1.

In accordance with a further variant, the semiconductor element,preferably one of the two active layers or both active layers, containsAl_(n)Ga_(m)In_(1-n-m)As, where 0≦n≦1, 0≦m≦1 and n+m≦1.

In accordance with a further variant, the semiconductor element,preferably one of the two active layers or both active layers, containsAl_(n)Ga_(m)In_(1-n-m)N, where 0≦n≦1, 0≦m≦1 and n+m≦1.

An LED semiconductor element according to an embodiment of the inventioncan advantageously be used for a radiation-emitting component since highluminances in conjunction with a comparatively small component size canbe obtained by means of the LED semiconductor element.

Furthermore, an LED semiconductor element according to an embodiment ofthe invention or the radiation-emitting component comprising the LEDsemiconductor element according to an embodiment of the invention can beused in particular for general lighting, for backlighting, for exampleof displays, or for projection applications.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention. It should be furtherunderstood that the drawings are not necessarily drawn to scale andthat, unless otherwise indicated, they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, advantages and developments of the invention willbecome apparent from the exemplary embodiments explained below inconjunction with FIGS. 1 to 4.

FIG. 1 shows a schematic cross-sectional view of a first exemplaryembodiment of an LED semiconductor element according to the invention;

FIG. 2 shows a schematic cross-sectional view of a second exemplaryembodiment of an LED semiconductor element according to the invention;

FIG. 3 shows a schematic cross-sectional view of a third exemplaryembodiment of an LED semiconductor element according to the invention;and

FIG. 4 shows a schematic cross-sectional view of a fourth exemplaryembodiment of an LED semiconductor element according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The LED semiconductor element 1 in accordance with a first exemplaryembodiment as illustrated in FIG. 1 comprises a firstradiation-generating active layer 2 and a second radiation-generatingactive layer 3, wherein the active layers are arranged one above anotherin a vertical direction, that is to say parallel to an emissiondirection and perpendicular to a main extension direction of the activelayers. A first semiconductor layer 5 of a first conductivity type, forexample a p-conducting semiconductor layer, and a second semiconductorlayer 6 of a second conductivity type, for example an n-conductingsemiconductor layer, are arranged between the active layers 2, 3.

The arrangement of the two active layers 2, 3 in the LED semiconductorelement 1 advantageously increases the amount of radiation generatedoverall. Since the dimensions of the LED semiconductor element 1 changeonly insignificantly by comparison with an LED semiconductor elementhaving only a single active layer and, in particular, the cross sectionof the LED semiconductor element is independent of the number of activelayers, more extensively the luminance is also advantageously increased.

The semiconductor element 1 comprises a contact zone 4, whichelectrically conductively connects the semiconductor layer 5 to thesemiconductor layer 6. Preferably, the semiconductor element 1 isprocessed in such a way that on at least one side flank part of thesemiconductor layer 5 and part of the semiconductor layer 6 areuncovered, whereby a first free region 9 not covered by semiconductormaterial and a second free region 10 not covered by semiconductormaterial are formed. The contact zone 4 extends from the first freeregion 9 to the second free region 10 and at least partly covers them.The contact zone 4 can contain a metal, a metal compound or aradiation-transmissive oxide (TCO) such as ITO.

Furthermore, in order to improve the electrical connection, the twosemiconductor layers 5, 6 can be embodied in highly doped fashion, suchthat an efficient tunnel junction with a low electrical contactresistance arises during operation.

The LED semiconductor element 1 comprises a rear side contact 7 disposedupstream of the active layers 2 and 3 in the vertical direction.Furthermore, the LED semiconductor element 1 comprises a front sidecontact 8 disposed downstream of the active layers 2 and 3 in a verticaldirection. Consequently, a vertically conductive component is formedwhich is distinguished by a comparatively homogeneous currentdistribution within the LED semiconductor element 1.

More extensively, the semiconductor element 1 can be arranged on acarrier element (not illustrated) on the side of the rear side contact7. In this case, the carrier element preferably contains an electricallyconductive material. By way of example, the semiconductor element 1 canbe a thin-film semiconductor element. In this case, the LEDsemiconductor element 1 is grown in particular on a growth substratedifferent than the carrier element and is subsequently mounted onto thecarrier element, which can be done for example by means of soldering,bonding or adhesive bonding, the growth substrate preferably beingstripped away from the LED semiconductor element. The rear side contact7 can simultaneously serve as a mirror, such that radiation componentsimpinging on the rear side contact 7 are reflected in a verticaldirection, that is to say in this case in the direction of a radiationcoupling-out side of the LED semiconductor element 1.

In the exemplary embodiment illustrated in FIG. 1, the active layers 2and 3 are preferably monolithically integrated in the semiconductorelement 1. By contrast, in the exemplary embodiment illustrated in FIG.2, an individual first layer stack I comprising the active layer 2 andan individual second layer stack II comprising the active layer 3 areconnected to one another in order to obtain the LED semiconductorelement 1. The production step of connecting the two layer stacks I andII is symbolized by the arrows.

A contact layer that forms the contact zone 4 after the connection ofthe two layer stacks I and II is arranged on the layer stack I. As analternative, the contact layer can be arranged on the layer stack II.The contact zone 4 is subsequently integrated into the LED semiconductorelement 1 between the first active layer 2 and the second active layer3. The contact layer contains an electrically conductive material.Furthermore, the contact layer is transmissive to the radiation of theactive layer 2 and/or of the active layer 3. The contact layerpreferably contains an adhesion agent, such that the two layer stacks Iand II are mechanically connected by means of the contact layer.

A rear side contact 7 can be applied to the layer stack I, while a frontside contact 8 can be formed on the layer stack II. The contacts can beapplied before or after the connection of the two layer stacks I and II.

In the exemplary embodiment of an LED semiconductor element 1 which isillustrated in FIG. 3, two layer stacks I and II are likewise arrangedone on top of the other, wherein the contact zone 4 is integrated intothe LED semiconductor element 1 between the layer stacks I and II. Thecontact zone 4 comprises a first region 4 a and a plurality of secondregions 4 b. The second regions 4 b are embedded into the first region 4a. Preferably, the second regions 4 b are electrically conductive. Theregion 4 a can be electrically conductive or insulating. As illustrated,the second regions 4 b can be embodied in the form of contact pads,wherein one second region 4 b is arranged on the layer stack I andanother second region 4 b is arranged on the layer stack II. The twolayer stacks I and II are connected to one another in such a way thatthe two regions 4 b lie one on another. The layer stacks I and II areelectrically conductively connected to one another by means of thesecond regions 4 b. Furthermore, the active layers 2 and 3 and thecontact zone 4 are arranged with respect to one another in such a waythat the active layers 2 and 3 are connected in series.

The two layer stacks I and II can be bonded onto one another by means ofthe second regions 4 b. In addition, the first region 4 a can contain anadhesion agent that mechanically connects the two layer stacks I and II.Preferably, the first region 4 a is transmissive to the radiationgenerated by the active layer 2 and/or the active layer 3.

The LED semiconductor element 1 illustrated in FIG. 4 comprises a firstlayer stack I and a second layer stack II disposed downstream of thefirst layer stack I in a vertical direction, wherein the contact zone 4is arranged between the layer stack I and the layer stack II. Thecontact zone 4 comprises a first region 4 a and a second region 4 b. Thefirst region 4 a and the second region 4 b are arranged between a firstconnecting layer 4 c and a second connecting layer 4 d. The firstconnecting layer 4 c and the second connecting layer 4 d preferablyserve to improve the charge carrier transfer between the layer stacks Iand II. By way of example, the connecting layers 4 c and 4 d can containa radiation-transmissive electrically conductive oxide (TCO) such asITO. Particularly preferably, the connecting layers 4 c and 4 d areapplied to the respective layer stack before the connection of the twolayer stacks I and II. One of the two layer stacks I and II isfurthermore provided with the second region 4 b, embodied in particularas a contact pad or contact web. The second region 4 b contains amaterial, in particular a metal, having a low electrical resistance,such that a comparatively good current flow across the contact zone 4can take place. The first region 4 a preferably contains an adhesionagent, such that the layer stacks I and II are mechanically connected inparticular by means of the first region 4 a.

The invention is not restricted by the description on the basis of theexemplary embodiments. Rather, the invention encompasses any new featureand also any combination of features, which in particular comprises anycombination of features in the patent claims, even if this feature orthis combination itself is not explicitly specified in the patent claimsor exemplary embodiments.

Thus, while there are shown, described and pointed out fundamental novelfeatures of the invention as applied to preferred embodiments thereof,it will be understood that various omissions and substitutions andchanges in the form and details of the illustrated apparatus, and in itsoperation, may be made by those skilled in the art without departingfrom the spirit of the invention. Moreover, it should be recognized thatstructures shown and/or described in connection with any disclosed formor embodiment of the invention may be incorporated in any otherdisclosed or described or suggested form or embodiment as a generalmatter of design choice.

1. An LED semiconductor element comprising: at least one firstradiation-generating active layer; and at least one secondradiation-generating active layer which is stacked above the firstactive layer in a vertical direction and is connected in series with thefirst active layer, wherein the first active layer and the second activelayer are electrically conductively connected by means of a contactzone, and wherein the contact zone is integrated into the LEDsemiconductor element between the first active layer and the secondactive layer.
 2. The LED semiconductor element as claimed in claim 1,wherein the contact zone has at least one first region and at least onesecond region.
 3. The LED semiconductor element as claimed in claim 2,wherein the second region is electrically conductive.
 4. The LEDsemiconductor element as claimed in claim 2, wherein the second regionis a contact pad or elongated contact web, and a material surroundingthe second region forms the first region.
 5. The LED semiconductorelement as claimed in claim 2, wherein the second region is arranged insuch a way that it produces an electrical connection between a firstlayer stack comprising the first radiation-generating active layer and asecond layer stack comprising the second radiation-generating activelayer.
 6. The LED semiconductor element as claimed in claim 5, whereinthe second region is applied on a surface of the first or second layerstack which faces the opposite layer stack.
 7. The LED semiconductorelement as claimed in claim 2, wherein a first layer stack comprisingthe first radiation-generating active layer and a second layer stackcomprising the second radiation-generating active layer have at leastone second region arranged in such a way that in each case two secondregions come to lie on one another when the two layer stacks are stackedone on another.
 8. The LED semiconductor element as claimed in claim 1,wherein the contact zone comprises a material that is transmissive tothe radiation generated by at least one of the first active layer andthe second active layer.
 9. The LED semiconductor element as claimed inclaim 1, wherein the contact zone comprises a transparent conductiveoxide (TCO).
 10. The LED semiconductor element as claimed in claim 1,wherein the contact zone comprises an adhesion agent.